A frequency hopping system is an example of a spread spectrum radio communication technique. By using a frequency hopping system, it is possible to secure a large number of independent communication channels in a limited frequency band by selecting an appropriate hopping pattern.
In a communication system of the frequency hopping system, a transmission side communication apparatus and a reception side communication apparatus hop a transmission frequency and a reception frequency in synchronization with each other in accordance with an identical hopping sequence (a hopping pattern). An example of a hopping sequence of the transmission and the reception frequencies used in this case a hopping sequence having a relatively short repetition period and a hopping sequence having a long repetition period in order to improve confidentiality. Whether the hopping sequence is short or long, an acquisition technique for specifying which frequency in the hopping sequence is a transmission frequency currently used by the transmission side communication apparatus and synchronizing hopping timing of a reception frequency with the transmission side communication apparatus is important in the reception side communication apparatus.
A frequency hopping spectrum communication system is divided into a low-speed frequency-hopping spread spectrum system and a high-speed frequency-hopping spread spectrum system according to a relation between a data rate and a frequency hopping rate.
The low-speed frequency-hopping spread-spectrum communication system is a system in which the frequency hopping rate is lower than the data rate. The low-speed frequency-hopping spread-spectrum communication system transmits a plurality of symbols at one frequency. As examples of such low-speed frequency-hopping spread-spectrum communication systems, JP-A-7-30516 discloses a system for determining switching timing for a frequency that synchronizes frequencies of a transmitter and a receiver by counting the number of bits of demodulated information; JP-A-5-191378 discloses a method of including frequency information to be used next in a packet, and synchronizing working frequencies of a transmitter and a receiver by using the frequency information included in the packet; and JP-A-11-8571 discloses a system for detecting a shift of a frequency by performing f-V conversion to set a frequency on a reception side.
On the other hand, the high-speed frequency-hopping spread-spectrum communication system is a system in which the frequency hopping rate is higher than the data rate. The high-speed frequency-hopping spread-spectrum communication system transmits one system at a plurality of frequencies. In this system, since one symbol is transmitted at a plurality of frequencies, it is possible to obtain a satisfactory characteristic with respect to a frequency selective fading such as a multi-path fading. As in example of such a high-speed frequency-hopping spread-spectrum communication system, JP-A-8-191260 discloses a high-speed frequency-hopping spread-spectrum receiver which can perform acquisition of a hopping sequence in a short time, and a correlator and a synchronizer used in the high-speed frequency-hopping spread-spectrum receiver.
However, in the methods disclosed in JP-A-7-30516 and JP-A-5-191378, a working frequency is determined from a signal, which has passed through a demodulator, on the receiver side. Consequently, an initial acquisition cannot be performed unless there is a signal that is demodulated by the demodulator correctly. There is, therefore, a problem in that, for the initial acquisition, a user has to wait until frequencies of the transmitter and the receiver coincide with each other and, moreover, has to take measures for outputting a preamble signal that is required only for synchronizing frequencies.
In the method disclosed in JP-A-11-8571, since a shift of a frequency is detected by the f-V conversion to set a frequency on the reception side, a user has to wait until a frequency generated from a receiving station is stabilized after the shift of a frequency is detected. When this system is used for a communication system having an extremely broad band, it is necessary to use a linear f-V converter over a broad band.
In the method disclosed in JP-A-8-191260, a method of reducing time required for conventional acquisition of high-speed hopping is described. However, there is a problem in that, even if the method is used, time for correlating signals is still required and, moreover, a plurality of circuits for correlating signals are required to complicate a circuit of the receiver.